IHV 


^^^^^^^^^M 


SB 732 

Iff 


^^^^^^^^^M 


.H4 

-i 
j- 

1 


^M 


■i 

; 

1 


1 




W^V\^^'"^^ °^^^\/'\ ^^K*\^^^% "-%^*' /'\ 


















N^r. 



'* ^^^ 

\by 






rO^ 



^^0^ 



^0 












.^^. 



V •- 



^^•^^. 



BULLETIN 



318-110-15h 



OF 

THE UNIVERSITY OF TEXAS 

No. 135 

ISSUED SEMI-MONTHLY 

SCIENTIFIC SERIES NO. 14 NOVEMBER 15, 1909 



Symptoms of Disease in Plants 



BY 



FREDERICK D. HEALD 

Professor of Botany in the University of Texas 




PUBLISHED BY 

THE UNIVERSITY OF TEXAS 



Entered as second-class mail matter at the postoffice at Austin, Texas 






Cultivated mind is the guardian genius 
of democracy, . . . It is the only dic- 
tator that freemen acknowledge and the 
only security that freemen desire. 

President Mirabeau B. Lamar. 



h2 L^ar-' ?'■■'■"! 
MAV 19 19ie 



SYMPTOMS OF DISEASE IN PLANTS 

It is luglily iiuportniit that nil persons cnoa.u'ed in <;cncral farming, 
truck gardening, fniit raising, or tioiiculture should Ik al»lo to recognize 
the presence of disease. In their ninte way plants tell ns when they are 
suffering from constitutional or contagions diseases if we are only able 
to interiiret their lang-^uige. It is evident that many people do not j'ecog- 
nize the presence of diseases, for it is not an nncommon thing to receive 
replies like the following to the letters of inquiry that are sent out over 
the state: "There are no plant diseases present in our locality"; "All 
the crops in our region are free from diseases" ; "No diseases in our 
country." 

The diseases that atfect plants may l)e divided into three different 
groups: first, those disturbances of nutrition which are due to unfavor- 
able conditions in the environment of the plant, or to the conditions in- 
herent in the ])lant itself; second, trouliles due to the presence of para- 
sitic plants, generally either bacteria or fungi, but sometimes to flower- 
ing ])lants which deform, stunt, or kill the affected plant by their pres- 
ence; third, those "caused by insect pests, which are in many cases very 
evident, and in other cases not easily detected. Many of these insect 
troubles are hardly to be classed as plant diseases, while many are as 
truly diseases as those produced by parasitic fungi. In the nature of the 
resulting symptoms, there are no sharp and fast lines that can be drawn 
between many insect and fungus troubles, and often a careful micro- 
scopic examination is necessary in order to diagnose the disease. The 
first thing of importance for the farmer is to detect the ]iresence of the 
disease and then to endeavor to find out its cause, nature, and probable 
outcome and the treatment which should be employed, by consulting 
those who have made the subject a special study. 

Before taking up the main part of this paper, which will deal prin- 
cipallv with the symptoms of diseases produced by bacteria and fungi, 
l)rief mention may be made of a number of flowering ])lants which are 
parasitic upon other flowering plants. 

The best example in this class is afforded by the various species oP 
dodder (Cvscuta), twining vines, devoid of chlorophyll or leaf green and 
possessing only rudimentary leaves. Dodder twines around the stems 
of the host plants much as a morning-glory twines around its support. 
The dodder is devoid of chlorophyll and is not attached to the ground 
by roots, so that it must ol)tain all of its food by robbing the host upon 
which it is growing. It does this bv sending its sucking organs or 



■4 University of Texas 

liavstoria into the stems around which it grows. One of our cultivated 
plants seriously infested with dodder is alfalfa. Seeds of four different 
.species of dodder may be found in alfalfa seed, and several of the species 




Fig. 1. — Dodder or "Love Vino," a serious parasite upon alfalfa. Original. 



are reported as doirg eonsidoral)l(' liarm in various parts of the alfalfa- 
growing territory. The dodder is a rampant grower, and soon kills 



Symptoms of Disease in Plants 5- 

alfalfa l)y rcibhinc" it of food and smothering it with its dense growth of 
intertangling stems. 

Many of onr forest and shade trees are attacked by another type of 
parasite, the mistletoe (Phoradendron sp.). This is a parasite which pos- 
sesses green coloring matter and is consequently able to supply a part of 




Fig. 2. — Trees badly infested with mistletoe (Phoradendron flavescens) 
Photograph in School of Botany collection. 



its own food. The mistletoe does, however, stunt and deform the host 
plant on which it is living, the amount of injury depending upon the 
extent of the infection. 

The distinction between true parasites and epiphijtes, which merely 
find lodgment on the surface of the plant on which they are growing,. 



6 



University of Texas 



should 1)G kept in mind, since several such plants are common in this 
region. The so-called '"long moss" {Bendropogon usneoides) which 
grows npon various f6rest and shade trees is a familiar example of such 
an epiphyte. This plant is not a moss as most people suppose, hut is a 
flowering plant, wdiich merely finds lodgment upon the branches of trees 




Fig. 3. — Tree completely covered witli "liall ]\Ioss" (Tillandsia reciirvata). 
Photogra])li t'ron: School of Botany collection. 



and does not rol) them of any of tlioir accumulated food material. The 
"short moss'' or "ball moss" {Tillandsia reciirvata) is equally familiar, 
and in this section of the state is more conspicuous upon park and shade 
trees occu])ying the liigher locations, while tbe ""long moss" is ])redomi- 
nant on trees occui)ying tbe valleys. Botli of these rpipJiytcs probal)ly 



SynipUjins of Disease in Plants 7 

cause injury to tlic trees wliieh liai'hor them, hut this can only be the case 
when the "moss" becomes especially abundant. By crowding tlie foliage 
of the tree, checking the development of yonng shoots, and harboring in- 
sect pests, the "long moss" or "short moss" may be detrimental to the 
life of the trees npon which tliey are growing, Imt they should never 
be looked upon as parasites. 

The following outline will give a survey of tlu' ])rincii)al symptoms of 
disease in ])lants, placing most empliasis on those diseases due to bacteria 
and fungi ; 

1. Discoloration or change of color from the normal. 

(a) Pallor. Yellowish or white instead of the normal green. 

(b) Colored spots or areas on leaves or stems. 

AVhitish or grey: mildews ; white rusts, etc. 

Yellow: many leaf spots. 

Eed or orange: rusts, leaf spots, etc. 

Brown: many leaf spots. 

Black: black rust, tar spots, etc. 

Variegated : leaf spots, etc. 

2. Shot-hole: perforation of leaves. 

3. Wilting: "damping off." "wilt," etc. 

4. jSTecrosis: death of parts, as leaves, twigs, stems, etc. 

5. Reduction in size: dwarfing or atrophy. 

6. Increase in size : hypertrophy. 

7. Eeplacement of organs by a new structure. 

8. Mummification. 

0. Change of position. 

10. Destruction of organs. 

11. Excrescences and uuilformations. 

Galls : pustules, tumors, corky outgrowths, crown galls, etc. 
Cankers: malformations in the bark generally resulting in an 

open wound. 
Punks or conchs and other fruits of fleshy fungi. 
Witches' l)rooms. 
Rosettes and hairy root. 

12. Exuflations. 

Slime flux. 

Gummosls: especially for stone fruits. 

Resinosis: especially for coniferous trees. 

13. Rotting: 

Dry rot and soft rof : the "gangrene" of plant tissue. 

Root rots: alfalfa, cotton, beets, cherry, etc., generally 
woodv or fleshv roots. 



8 University of Texas 

Stem or trunk: dry rot of trees; rot of modified stems 

like rliizomes, Ijulbs, or tubers. 
Buds. 
Fruits: flesliy fruits of various kinds. 

It is quite common to find the foliage of trees in the prairie regions 
showing a pronounced yellow cast instead of the normal green color. 
This yellowing is especially prominent in the regions where the soil is 
decidedly alkaline or where there is a seepage of ground water from high 
irrigation ditches to a lower level, or where both of these conditions pre- 
vail. The conspicuous sickly yellow corn plants that may be found in 




Fig. 4. — Powdery mildew (Mierosphaera sp.) of tlie lioney suckle. The leaf 

nearljr covered by the fungus shows groups of minute black bodies, 

the spore -fruits or perithecia. Original. 

the early part of the season upon flooded fields or in water-logged soils 
is another example of this marked symptom of disease. The lack of 
sufficient iron in the soil will also cause a plant to pass into what is 
technically termed a chlorotic condition, that is, the leaf green or chlo- 
rophyll is either not developed at all or only to a slight extent. These 
chlorotic plants can often be restored to their nornial condition by supply- 
ing them with iron. The occasional white plants that appear in corn 
fields are some of our plant "albinos," and are suffering from rathe]' 
complex disturbances of their nutrition. Anotlier physiological trouble, 
the "mosaic disease," common in the tomato and the tobacco plants, is 



Sympiom.s of Disease in Plants 9 

characterized by the mottled appearance of tlie leaves, or, in other words, 
the leaf is somewhat variegated with yellow and green. 

When the leaf shows colored areas or spots that are more or less definite 
and circnmscribed, and different from any normal coloration we may be 
fairly certain that it is suffering from cither an insect or fungus disease, 
with the ])rcsum]>tion in favor of the latter. 




Fig. 5. — Crimson rambler rose showing powdery mildew (Sphaerotheca pannosa) 
on stem and leaves. The leaves are curled and reduced in size. Original. 



White patches more or less definite or somewhat diffuse are generally 
due to either some of the "Powdery Mildews" or to the "Downy Mil- 
dews." We have a good illustration of this symptom in the powdery 
mildews of the apple, peach, cherry, grape, gooseberry, wheat and other 
grasses, the elm, catalpa, mesquite, the lilac, honeysuckle, crimson ram- 
bler and other roses, golden glow, etc. In all of these cases the fungus 



10 



University of Texas 



tliat causes the disease Lives on the surface of the host plant, or is what 
may be termed an external parasite. During the first part of the season 
the surface of the leaves ot stems of the host plant is covered with an in- 
terlacing tangle of minute wliite fungus fiUimcnts. and the powdery ap- 
pearance is duo to the production of large numbers of minute repro- 




Fig. 6. — Twig from a young cherry tree infected with powdery mihlew. {Podo- 

sphaera oxycanthae). The mildew is very abundant on the lUider 

surface of the leaves and causes the leaf blades to roll. Original. 



ductive bodies or .spores, wliicli Ijccome lioaiied up on tlie wliite patches. 
Later in the season many of these "Powderv ^fiblews" |)roducc winter 
fruits, wliicli sbow as small black bodies scattered over the wbite ])atche3 
or grouped in special clusters. (Fig. 4.) 

In some of the ''Powdery Mildews" tbe presence of the fungous para- 



Symptoms of Disease in Plants 



11 



site causes more or less discoloration of the atfected leaves togellier with 
more or less curling or rolling. This is especially marked in tlie powdery 
mildew of the cherry. (Fig. G.) In the powdery mildew of many roses, 
the leaves are cui-led and at the same time stunted or dwarfed in si;^e. 

In the downy mildew of the grape, wliich is an internal parasite, wo 
often find the under side of tlu' leaf covei'cd with irreuiilar wliitisli naTclies 




Fig. 7. — Leaf of grape vine showing the coiisjjicuoiis patches of downy miklew 
(Peronospora viticohi) upon the under surface. Original. 



which arc formed hy tlio spore-producing parts of the fungus that einerge 
from the interior of the leaf. It may be mentioned in this connection 
that the upper side of the leaf, directly over these spots is at first a yellow 
color, but later changes to a dark brown. When the disease is very jircva- 
lent, the leaves may turn brown throughout, shrivel somewhat, and fall 
from the vines, or thev may be cast while thev are still more or less 



12 



University of Texas 



mottled with green, yellow, and brown areas. Other downy mildews 
showing somewhat the same symptoms may be found on cucumbers, let- 
tuce, spinach, and onion among garden vegetables, and upon some of our 
forage crops like millet, clover, and alfalfa. 

In some diseases which are not due to "Powdery" or "Downy" mil- 
dews, the appearance of white patches upon the under surface of the 
leaves with yellow areas upon the opposing surface is characteristic. This 
is especially noticeable in a serious leaf-spot disease of the parsnip. 
(Fig. 8.) 




Fig. S. — Poiiion of parsnip leaf affected by fungus (Cercosporella sp.), which 
produces conspicuous white spots upon the under surface. Original. 



The "white rusts" generally show more definite wdiitish pustules which 
eventually rupture and allow the escape of the spores. The white rusts 
may be observed upon cress, the sweet potato, morning glory, turnips, 
radish, cabbage, horse-radish and related plants, and also upon the leaves 
of the oyster plant. The white rust of the oyster plant may be so severe 
as almost to kill the tops and thus greatly reduce the development of the 
roots. 

In some diseases the affected spots are of a grayish color, either uni- 
form throughout or with margin or center of a different color. In a leaf 



Symptoms of Disease in Plants 



13 



disease of the elm, the leaf is covered with irregular grayish patches with 
dark or black, somewhat papillate centers, while parts of the same leaf 
may be brown and dead. In the leaf-spot of the common strawberry the 




Pig. 9._Leaves of oyster plant affected with white rust (Albugo tragopogonis). 

The tips of some of the leaves have turned brown and have 

begun to curl. Original. 



fungus causes numerous circular grayish spots surrounded by quite defi- 
nite purple borders. A disease of the prickly pear, called anthracnose, is 
not uncommon in the Southwest. Circular grayish, slightly sunken, 



14 



Universittj of Texas 



patches, one-half inch or more in diameter, witli dark or black centers, 
are qnite characteristic of this disease. In an anthrocnose of the box. the 
mature spots are grayish, one-fourth to one-half inch or more in diam- 
eter, and surrounded by a narrow brown border, or show more or less con- 
centric zonation. In the early stages of the disease the spots are yellow 
and turn gray with the development of the disease. 

Many fungi tbat inhal)it leaves have a localized effect and produce more 




Fig. 10. — Leaves of the cork-winged elm showing abundant scab spots 
(Gnomonia nlmea ) . x 2. Orioinal. 



or less circular or slightly irregular spots of a nuirked yellow color. In 
such cases the disease is generally characterized as a "leaf-spot.'' Symp- 
toms of this kind may be noted in the leaf-spots of tbe oak and violet, and 
in the antJiracnose of cucumbers and melons. Sometimes the change in 
color becomes more general, and the whole leaf turns yellow and falls 
from the tree as in the early summer stage of apple scab. 

Another marked symptom of disease is the occurrence of red or orange 



i:^ijiit [itdiiifi of Disease in /'hi ills 



15 



sjiots or [lustulc's. The "Ihists" of our cereals and otlier crops received 
their name from the fact that in the early stage of the attack, reddish or 
brownish spore-producing- pustules give the stems or leaves a rusty ap- 
pearance. Sometimes these rust spots may l)e few in miiiihcr, at other 
times so numerous that almost the entire surface of leaf or stem is cov- 
ered, and thus the vigor and productiveness of the crop is seriously im- 
paired. In all of our cereal rusts, black pustules apjiear on leaves and 
stems as a later growth. I'he red pustules always predominate and give 
character to the disease in the "orange rust" of wheat and the "crowned 
rust" of oats, while the black stage or black rust is the conspicuous stage 




Fig. 11. — A segment of priekly pear showing several anthracnose spots 

( Gloeosporivun sp. ) . Photograph (by F. D. Heald 

and F. A. Wolf. 

in the "stem rust" of wheat. Some of the other cultivated plants affected 
by rusts are the clovers and alfalfa, cotton, beans and peas, beets, aspara- 
gus, plum, peach, cherry, tig, cottonwood and willow, roses, carnations 
and chrysanthemums. The cluster-cup stage of many of the rusts pro- 
duce fairly large, well-defined orange-colored spots in many cases. This 
symptom is well illustrated in the well-known "cedar rust" of apple 
leaves, and in the cluster-cup stage of the "crowned rust" of oats on the 
leaves of the buckthorn. Deeper red or even purple colored sjwts or ex- 
tended areas indicate the presence of a leaf parasite in many cases. This 
is often noticed in the early stages of the shot-hole diseases of the cherry 



16 



University of Texas 



or the peach, and is especially characteristic in the bacterial blight of 
sorghum, Johnson grass, or Kaffir corn. In the last-mentioned case the 
dark red or purple patches are generally somewhat elongated and more or 
less irregular. The leaves of dandelion affected by rust generally show a 
reddening of the whole leaf, while the under sides of blue-grass lenves 
affected with the powdery mildew frequently show spots of a deep purple 
color. 




Fig. 12. — Violet leaves affected with leaf-spot ( Alternaria violae) . Pliotograph 
iby F. D. Heald and F. A. Wolf. 



The pods of beans frequently become conspicuously "rusted." The 
presence of rust-colored, sunken patches, either scattered or very almn- 
'dant, is characteristic of the so-called anthracnose of beans. The disease 
is esj>ecially noticeable on the wax-podded varieties, and the majority of 
farmers and gardeners speak of the trouble as "rust," although the fungus 
which causes the trouble does not belong the the true rust fungi. 

Definite brown areas, either small or somewhat extended, are quite 
cliaracteristic of spot diseases of either leaves or fruits. The leaf-spots 



Symptoms of Disease in Plants 



17 



of apple, pear, and plum are common diseases in this and adjoining 
states. As a result of these troiiljles, the leaves beoin to drop and a pre- 




Fig. 13. — Leaves of Johnson grass affected with hacteriosis (Bacillus sorghi). 
Photograph by F. A. Wolf. 

mature defoliation is the result, thus sending the tree into winter rest 
with an insufficient supply of reserve food for the growth of the follow- 



18 



University of Texas 





^K. y^ 




M^^ 


HL^' ' 


^ 


^d^SSmSM) \ \ \}^^MSBi 




aSsaBB**^^^^^ 






*T^fT^ 


^^^^^ ^ X^^RilH^^^^^^llBr^ 




^ 









Fig. 14. — Apple leaf from the under surface showing conspicuous patches of 
rust ( Gymnosporangium juniperi-virginianae) . Original. 




Fig. 15. — Pods of hlack wax Jieans affected with anthraenose, commonly called 
"rust" (Colletotrichum lindemuthianuni) . Original. 



Sympioms of Disease in Plants 



19 



ing season. As a result of tJie leaf-spot, Ben Davis trees are often de- 
foliated when other varieties are in full foliage. The leaves of tomato 
plants often show an abundance of small brown spots. As these spots 
become more plentiful, the intervening areas turn yellowish, the leaf 
begins to shrivel and curl and finally drops off. JMany gardeners d(j not 
recognize this behavior of tomato leaves as a symptom of disease, since 
it is of such common occurrence. The early blight of potatoes is char- 
acterized by brownish spots on the leaves that often show more or less 
of a concentric zonation. As the fungus spreads, the spots may coalesce. 




Ylg. 10. — Leallets from tomato plants afi'ected witli leaf-spot (Septoria 
]yco.persici) . Original. 

and thus the entire leaf will be killed. Sometimes the brownish color 
is not confined to definite spots, but is more general as in the difi^use form 
of apple-scab, which shows upon either the under or up|oer surface of the 
leaves. 

The definite scab spots on the fruits or leaves of apples, pears, or 
peaches may be gray, brown, dark olive green, or even almost black. More 
or less deep cracks or fissures often appear in later stages of scab. In 
the Ascochyta blight of the pea, the leaves and pods become covered with 



30 



University of Texas 




Fig. 17. — Peaches showing abundant brown spots sometimes called "peach 
freckles" (Cladosporium carpophiliim) . Original. 

more or less circular brown areas of dead tissue. The brown patches 
may coalesce to form extended dead patches on either leaves or pods. A 
careful examination of the patches will show minute black specks, the 
fruits of the fungus, scattered over the middle of the spots. Many other 




Fig. 18. — Pods of peas showing difi'erent degrees of abundance of the Ascochyta 
(blight (Ascochyta pisi). Original. 



Symptoms of Disease in Plants 



21 




Fig. 19. — Leaf-spot of rhubarb, showing shot-hQle effect and concentric zonation. 

Original. 




Fig. 20. — Tar-spot (Rhytisma acerina) of willow and silver maple. Original. 



<5tO 



University of Texas 



examples of a somewhat similar character may be mentioned, as the leaf- 
S2>ots of beets, watermelon, maple, mulberry, chrysanthemum, rose, rhu- 
barb, etc. Other troubles, such as spray injury, or sun-scald, may be 
indicated by brownish discolorations, but in the majority of cases this 
sjmiptom indicates a fungus disease. 

The appearance of black spots upon stems or foliage is not an uncom- 
mon symptom of disease. One of the most striking cases of this char- 
acter is in the so-called "tar-spots" of our maples, willows, and live oaks. 
Leaves affected by this trouble show large, irregular l)lotches of blackened 
tissue — in fact, they look almost as if they had been spattered with drops 
of tar. When these spots become abundant, the intervening tissue of the 




Fig 21. — Fly-speck fungus (Leptotliyriuin pomi) on an ai>ple. Original. 

leaf turns yellow, and the leaves are drop]ied quite early. In severe epi- 
demics considerable injury may result from these "tar-spots.'' In all of 
our cereal rusts, the winter stage is indicated by either small black cov- 
ered pustules or by elongated black streaks having a somewhat powderv 
appearance. The winter spore pustules of the orange leaf rust of wheat 
are small and covered, and are generally confined to either leaf blade or 
sheath, while the winter spore pustules in the stem rust of wheat, rye, 
or oats are in the form of elongated naked streaks, wliich are more numer- 
ous on the stem than on any other parts of the plant. In a leaf disease 
of the honey locust, the leaflets often show numerous small l)lack spots 
on the under side: in many cases tliese black spots become so numerous 



Syiitpfonis of Disease in Phnils 



23 




24 



University of Texas 



that they coalesce and cover the entire surface of the leaflet, hence the 
common name "black leaf" disease. Another fungus trouble that is quite 
common in orchards is the so-called "fly-speck" fungus of the apple. 
Fruits attacked by this fungus show small clusters of circular black spots 
which on account of their characteristic appearance have suggested the 




bo 

5 



common name. This fungus is more common and evident on the yellow 
skinned varieties. The "sooty" fungus produces larger, and more diffuse, 
smoky brown, or almost black, irregular patches upon the epidermis of 
apples. 

In some of the anthracnoses the affected areas are circular in outline 



Symptoms of Disease in Plants 



25 



and show concentric circles of minute black spots, the fruiting pustnles 
of the fungus which causes the trouble. It has been shown that this 
zonation is flue to the alternation of light and darkness. The anthracnose 
of the box and of some of the squashes and gourds, and also the an- 
thracnose of the tomato afford excellent examples of this symptom. 

Leaves which show circular or sometimes irregular perforations have 
probably been affected by some fungus parasite. In many cases leaves 
so affected will show brown spots of dead tissue which have not yet 
dropped out. Several diseases have been called "shot-hole" diseases on 
account of this peculiar effect upon the foliage. Notable among these 
troubles is the shot-hole disease of the cherry and plum. This is some- 




Fig. 24. — ^Sliot-hole disease ( Cercospora sp.) of tlie Virginia creeper. Original. 



times called "rust" by orchardists, but the true cherry rust is an entirely 
different disease. Nearly all of the English Morello cherry trees in bome 
sections of the conntry have been killed during the past few years by this 
so-called "shot-hole" disease. In this disease of the cherry the leaf tissue 
adjacent to the affected spots frequently shows a marked purple colora- 
tion, while at certain stages in the development of the disease the inter- 
vening areas may be distinctly yellow in color. Sometimes the leaves fall 
from the tree before the perforations are complete, while in other cases 
the leaves still hanging upon the tree will be found to contain numerous 
perforations. 

Both Cercospora and Pliyllosticta spots of the Virginia Creeper fre- 



26 



University of Texas 



quently sliow a marked shot-hole effect. The perforations are so numer- 
ous that in some cases they might easily be mistaken for the work of 
insects. Such a Ccrcospora spot is shown in the accompanying illustra- 
tion. (Fig. 24:.) 

A sudden wilting of a plant which can not be explained by dry condi- 
tions of soil or air is often due to the attack of a parasite. Both seedlings 
and mature plants exhibit this symptom, and the loss which is occasioned 
by diseases having this attendant symptom is often enormous. In a 




Fig. 2.5. — Shot-Iiole disease (Cylindrosporium padi) of the plum. Original. 



seedling the disease in Avhieh this symptom is prominent is often called 
"damping off." The young plant drops dead, as it were, for a fungus 
parasite has entered the stem at about the ground level, and has struck 
a vital Ijlow, interrupting essential physiological functions. A disease 
of cotton seedlings of this nature is prevalent througliout the South, and 
is known to the planters as "sore-shin." A disease of the squash and 
related forms called "wilt" is not uncommon. In this trouble the para- 
site enters tlic stem and grows rampantly in the water vessels or ducts 



Syinpfoiiis of Disease in Plants 



27 



until they are completely plugged, and the supply of water to the foliage 
is interrupted. Often the first indication of the presence of this disease 
is the sudden wilting of the plant, no previous warning symptom having 
made its appearance. 




Fig. 26. — '•Damping off" or '-sore-shin" of cotton seedlings (Rliizoctonia sp.), 

Each seedling shows a dead brown area at the ground level. 

Photograph by F. A. Wolf. 



In some diseases, the death of parts of the plant, as leaves, twigs, 
stems, flowers, etc., is the first symptom that is observed. In the fire- 
blight of pears and apples the young terminal twigs with their leaves and 



28 



University of Texas 



flowers are killed and even large branches are involved and succnmb. 
The leaves turn brown and dry up while still hanging upon the tree, and 
the twig will be found to be dead down to a certain point, which marks 
the advance of the bacterium which is the cause of the disease. In this 
disease small gummy drops which consist of myriads of these bacteria, 
often ooze out from the dead bark. Sometimes the blossoms of plums 
turn brown and die prematurely as if blasted by frost or fire. This 




Fit 



-Pear trees killed by "fire bliglit.'' Tree in the foreground covered with 
dead leaves, brown and curled. Photograph by F. A. Wolf. 



blighting of the blossoms is due to a different organism, the brown rot 
fungus, which causes the rotting of peaches, plums, cherries, and some- 
times apples later in the season. 

In many diseases a more or less marked reduction in size, a dwarfing 
or atrophy of the whole plant or some of its organs, is evident. A 
crippled and deformed individual or a poorly developed organ, with im- 
poverished nutrition, is the result in many coses. The dwarfing or re- 



Symptoms of Disease in PlanU 



39 



m 

" to 



^ 3 













30 



University of Texas 



duction in size of apples by attacks of scab is often prominent. Many 
apples seriously affected do not reach over one-fourth the natural size, 
while others are gnarly and deformed, owino; to localized atrophy of the 
tissues of the fruit. The same may be said of apples aifected by "cedar- 
rust," although perlmps the effect is not as striking as in apple scab. 




In a disease of the peach, known as "little peach," a marked reduction in 
size of the fruit is one of the attendant and characteristic symptoms. 
Every farmer is familiar with the shrunken and shriveled appearance of 
wheat from badly rusted fields. The energy of the plant is sapped by the 
rust fungus, and as a result the grains are poorly developed, lessening 



Syirrptoins of Disease in Plants 



31 




32 



University of Texas 



the yield and qualit}'- of the crop. The dwarfing of stem, leaves, roots, 
flowers, or flower parts, or even the complete arrest of seed development, 
is often the result of the inroads of some parasitic fungus. 

While a large number of fungi produce more or less extensive atrophy 
or dwarfing of parts of their host, others cause abnormal enlargements of 
organs or parts of organs. The so-called "plum pockets" or "fools" is a 




Tig. 31. — Wheat grass affected witlh ergot ( Claviceips purpurea). Original. 



•striking illustration of hypertropliy. In this disease the outer layers of 
the fruit become thick and fleshy, while the pit remains undeveloped, 
causing the modified fruit to be hollow, hence the other common name of 
"bladder plums" which is sometimes employed. The flower parts of the 
radish and related plants are frequently enlarged and deformed as a re- 
sult of the attacks of white rust, while the catkins of some of our trees 



Syinplonis of Disease in Plants 



33 



shoM' striking modilications of size and cdlor due to the stininlatino' 
effect of their fungns guests. The twigs, petioles, and leaves of our com- 
mon ash often show orange-colored enlargements due to the growth of the 
cluster-cup stage of a rust fungus. The vegetative parts of plants, as 
stems, leaves, and roots, often exhibit peculiar enlargements, but most of 
these may be considered under the head of excrescences and malforma- 
tions. 

It sometimes happens that a parasitic fungus produces a new structure 
which takes the jilaee of some normal organs of (he plant, as in the so- 




Yig. 32. — ^Rye grass afl'ected with ergot (Claviceps ,purpurea ) . Original. 

called "ergots'' of our wild and cultivated grasses and rye. These ergots 
are horny resting bodies or sderotia of the fungus that occupy the same 
position as the kernel or seed, and appear only with the destruction of 
the seed-producing structure. These ergots not only affect the seed pro- 
duction of the host plant, but they contain poisonous principles which 
produce serious disease when ergot-infested hay is fed to cattle. 

The production of "mummies'' is another characteristic symptom of 
disease in fruit trees. Apples that are l^adly affected with either brown 
rot or black rot often dry up slowly and remain hanging on the tree over 



34 University of Texas 

winier in a more or less hard, shriveled condition. These mummies at 
the end of the winter period of rest may produce a crop of spores v/hich 
Yvill spread the disease. This spore-production may take place while the 
mummies are still hanging on the tree or on the fallen mummies. The 
formation of mummies is especially common in plums and peaches that 
are affected with the brown rot. It must he at once evident that mum- 
mies should be destroyed in order to prevent production of new crops 
of spores and thus to lessen the ravages of these rot-producing diseases in 
an orchard. The practice of allowing rotting plums or peaches to fall to 
the ground and remain beneath the tree should certainly he discouraged. 
Change of position is a symptom of disease that is sometimes over- 
looked. This is well illustrated in some plants which are affected with 




Fig. 33. — Normal apple and "mmiimy'" produced by the black-rot fungus 
(Spliaeropsis maloruni). Photograph 'by F. D. Heald and Leva Walker. 

a rust fungus, in the cluster-cup stage. Some of the spurges which are 
normally more or less prostrate or creeping become erect when attacked 
by a fungus of the kind mentioned. The common garden weed known 
as "pusley" grows normally in a prostrate condition, but when it is at- 
tacked by the "white rust," many of the seriously affected branches be- 
come more or less erect or ascending. This same symptom is present 
in some tree diseases, in which normally horizontal limbs or branches 
become more or less erect. 

As a result of the attacks of a fungus parasite a complete destruction 
of organs may result. This effect is well illustrated in the majority of 
our cereal smuts. In the loose smut of wheat, for example, the complete 
inflorescence is destroyed, the glumes and other flower parts being re- 
duced to a powdery mass of black material, the smut spores, Avhich 



Symptoms of Disease in Flanis 



35 



finally drop away, leaving nothing Imt the bare central axis of tlie head. 
The same effect with bnt little deviation may be noted in the naked and 
covered smuts of barley, and the loose smut of oats, while in the kernel 
smut of sorghum, the kernel smut of oats, and the bunt of wheat, it is 




Fig. 34. — 'Winter barley afl'ected with loose or naked smut (Ustilago nuda). 

Original. 



the berry alone M'hicli is destroyed, the surrounding parts remaining in- 
tact. In the last mentioned cases the berry or "seed" may show an in- 
crease of size or a modification of form Avith the complete destruction of 
all its tissue except a surrounding membrane which serves to confine the 



36 



University of Texas 



mass of smut spores. When the membrane is rui)tured, the interior brown 
mass crmnbles to powder, since it is simply a loose aggregate of spores 
which have been formed at the expense of embryo and endosperm. In 
■sorghum affected with the kernel-smut each grain or kernel of the dis- 
eased head or inflorescence is destroyed and the whole head has a much 
more compact form than is characteristic for the normal inf!orescence. 
AVheat fields in which l)unt or stinking smut is present emit a character- 
istic odor, and an examination of affected heads will show that each 




Fig. 35. — Winter barley affected Avitli covered smut (Ustilago hordei), 



smutted head has all of its "berries" destroyed, and further that all 
heads from a given "stool'' or plant are invaded and destroyed if the 
fungus is present in any. Wheat from fields infested with bimt is often 
dark in color due to the immense numbers of spores lodged upon the sur- 
face of tlie kernels and especially collected in the ■'lirush'' or tuft of liairs 
at the distal ends of the berries. In many cases wheat is so seriously 
infected with bunt that the yield is enormously reduced and the market 
value of the actual vield materiallv decreased. 



Symptoms of Disease in Plants 



37 



As a result of Iho stiimilalinti- effect of a fungus yiarasite, parts or 
organs of the host phmt may exhibit variously formed excrescences or 
malformations. These malformations may he in tlie form of pustules 
or small hlister-like eknations upon the surface of the lenf or stem, as 
in the "white rusts,'"' or the blistered areas may be quite extensive and 
cause more or less deforming and rolling of the leaf as in peach 'leaf- 
curl." Sometimes the ahnormal formation is in the form of a smui 




Fig. 30. — Normal and smutted heads of sorghum; normal kernel and smutted 
kernel (Sphacelotheca sorghi). Original. 



mass or tumor which, when mature, is filled with a brown or black 
powder, the spores of the fungus. In the smutted corn plant these 
tumors may occur on any part of the plant, not a single aerial organ 
l>eing exempt, although the majority of people think of corn smut as 
affecting only the ears or tassels. The head-smut of sorghum simu- 
lates corn smut somewhat in general external appearance, l)ut in this 
case the whole inflorescence is destroyed by the time it emerges from the 
leaf sheath. 

The so-called "cedar apple" is a good illustrarion of a fungus gall. 



38 



University of Texas 




Fig. 37. — Bunt or stinking smut of wheat, (a) whole head affected with smut; 

(,b) smutted grains; (c) normal grains; (d) smutted grain broken to 

show smut; (e) normal grain divided in the middle; (f) spores 

much enlarged; (g) germination of a spore. From a 

drawing by Venus W. Pool. 



Symptoms of Disease in Plants 



39 



These brown cedar apples may be present on tlio cedar trees in largo 
numbers in case of trees that stand adjacent to an apple orchard, and 
they may vary in size from about that of a radish seed to nearly two 
inches in diameter. Badly affected cedar trees may be so covered with 
these galls that many of the branches droop with tlie added weight ]ike 




Fig. 38. — Heads of wheat wliieh have heen dwarfed and blighted by scab 

(Fusarium culmorum). The head overgrown iwith the cottony 

mycelium was kept in a damp chamiber over night. Original. 



the branches of a heavily laden fruit tree. With the onset of the first 
warm rains of spring, the ''cedar apples" produce numerous orange- 
colored projections which stand out in all directions and thus give rise 
to the characteristic gelatinous rosette-like structures. I have known 
peoph:" to admire the "cedar apples" as tlie true fruit of the cedar, or to 



40 



University of Texas 




w 



Si/mptoniM of Disease in Plants 



41 



look upon the gelatinous rosettes as the flowers of the cedar, little realiz- 
ing that the structures are not nonual, but due to a parasitic fungus 
which may often seriously affect the lilc of the cedar and greatly impair 
the productiveness of adjacent a|)ple orchards. The gelatinous projec- 




Fig. 40. — Head siimt ( Spliaoelotheea reiliana) of sorghum. Original. 

tions soon dry up and leave the old dead galls hanging up(m the 
branches. 

Here may also be mentioned tlie disease of plums and cherries known 
as "black-knot." This disease is often not noticed until the conspicu- 
ous black enlaroemcnts liecomc evident later in tlie season. The enlarge- 



42 



University of Texas 




Fig. 41. — ^Cedar apples (Gymnosporangiiim juniperi-virginianse) on the common 
ceaar, one mature and the other from the previous season. Original. 




Fig. 4-2. — Cedar apples showing the production of the gelatinous sori. Spring 

stage. Orioinal. 



Symptoms of Disease in Plants 



43 



ments are somewhat irregular, roughened, and generally extend for some 
distance along the length of the affected twigs. When young or in the 
early S2>]"ing the newly formed knots are olive green in color. In this 




Fig. 43. — Black knot (Plowriglitia morbosa) on the Burbank plum. Original. 

condition they produce a crop of spores that spreads the disease, while 
later the older knots produce a second kind of spores. The fungus 
that causes the black^knot lives perennially in the twigs and branches 



44 



University of Texas 




Fig. 44. — Potato showing;' ileep sc.ib and surt'aco spots (Oos|)ora sca'bies). 

Original. 

and consequently new growths appear each year unless the affected limbs 
are pruned off'. In several sections of the United States plum orchards 
are seriously affected, and it is not uncommon to find the wild plums in 
many regions covered with malformations of the character described. 




Fig. 45. — Crown-gall on the iieacli. Original. 



Syiiiiitoiiis of Duoasc in Flanin 



45 



In some fiuiii'ons diseases the iiiair()nii;!ti<ins do not show as pronounced 
enlargements. This is true in our eoinmon potato seah, in which irreg- 
ular roughened areas appear over the surface of the potato. An ab- 
normal development of the corky tissue may result in a slight elevation 
of the scah patch, or in tlu^ deej) form there may l)e more or less cor- 
rosion and destruction of tissue. 




Fig. 46. — Crown-gall on the raspberry. Original. 



The so-called crown-galls wliich may l^e called our "vegetahle cancers" 
are due to the stimulating effect of some organism in the majority of 
cases. In these cases irregular al^normal enlargements appear at the 
crown or even upon some of the roots as in the raspberry, blackberry, 
grape, peach, apricot, and apple. The general similarity of the abnormal 
formations produced upon different hosts in these crown-gall diseases is 



46 



University of Tearis 



shown in the accompanying ilhistrations. Tho peach tree from which the 
photograpli was taken had been killed as a result of the disease, while 
the raspberry plants which frequently showed large galls gave little in- 
dication that they were seriously affected by the disease. Some of th'j 
vegetable cancers are apparently infectious, while others do not appear 
to be caused by the presence of a parasite and must be caused by a 
"rebellion of certain body cells" of the plant, just as human cancers are 
produced by similar deviations from the normal. 




Fig. 47. — Two young apple trees affected with crown-gall. Original. 



The club-root or finger-and-toe disease of the cabbage and other crucif- 
erous plants should also be included in this category, while the tubercles 
jDroduced upon the roots of various leguminous plants, either wild or cul- 
tivated, are not injurious, but are decidedly beneficial since the presence 
of the bacteria in these tubercles makes it possible for the legume to util- 
ize the free nitrogen of the air. These tubercles on the roots of our 
common garden peas are especially large, and our native Texas Lupine 
or Blue-bonnet has root-tubercles of pronounced size. 



Syinpioni.s of Disease in Plants 



47 



Tlio name "canker" is applied lo a niairoiination in tlio bark of trees 
wliicli generally resnlts in an open wound. In the younger stages of a 
canker the bark may be only slightly different from the normal, some- 
times being sunken, sometimes showing a more tnmid condition. The 
canker area may also show dillVrcnces of color, and with tlie advance of 
the disease the bark becomes roughened and begins to break and peel 
away. The destruction of the cortex or Ijark may thus leave an open 







M ' 






W" 




f§} \ 




■ 


^^^^mrr 


1 




■ 


^^^^IHr' 



Fig. 48. — ^L'aiiker (iii a.pple limb. Original. 



wound which exposes the wood, while the parasite itself penetrates the 
wood deeper and deeper, or other wood-rotting fungi gain an entrance 
and help complete the destruction. There are at least six different canker 
producing fungi which groAV upon apples trees. The same organism 
which produces the canker may also cause a rotting of the fruit. This is 
true in the bitter rot canker, the black rot canker, and the Pacific coast 
canker, while the organisms whicli cause European canker, the Illinois 



48 



University of Texas 



apple tree canker, and fire-blight canker do not attack the fruit. In some 
cases open wounds on trees due to sun-scald may be confused with 
cankers due to fungi, but in general a careful observation of the location 
will reveal the difl'erence. Canker-producing fungi may, however, enter 
sun-scald wounds and continue the destruction. A bacterial canker of 




Fig. 49. — Braiu'hcs of apple limbs affected witli Illinois canker (Nunimularia 

discreta). (a) Old canker; (b) mature canker. Pbotograph by 

F. A. Wolf and F. D. Heald. 



plums has Ijeen observed especially u])on two varieties, the Wickson and 
\Yhitaker. These ])liini cankers are on the smaller branches and are pro- 
duced by radial (is^u^es in the bark which make more or less open 
wounds due to the destruction of tissues and the relief of the transverse 
tension. 



Si/mploiiis of Pixf'dsc III Phnils 



49 



It is not uiu'oninion to litul upon the li'iiuk oi- linihs of forost, orna- 
mental, or orchard trees the fruits or sporo/iliores of some of the higher 
fungi. Some of these common foiiiis arc toadstool-like in appearance, 
while others are in the foi'm of Iti-ackcts, I he so-called l)raeket fungi. The 
latter are often called "punks" or "conclis" hy hiiuhermen and foresters. 
Some of these higher fungi only grow in wood or hranches that are dead 
or in a languid condition, Avhile others may gain an entrance through 
open wounds into healthy linilis and begin the work of destruction, while 
still others are more virulent and do not require either of the above con- 
ditions. Tn general, these fruits of fleshy fungi upon a woody plant may 
be considered indicators of disease, and they sliould ))e destroyed as soon 




Fig. 50. — Plum twigs showing cankers produced ,by bacteria. Original. 



as they appear to jjrevent the spread to other trees. The wood of a tree 
is always well pervaded by the vegetative body of the fungus and par- 
tially disintegrated before these sporophores appear, and although the 
life of the tree is generally doomed it may often be prolonged by proper 
care. When orchardists recognize the fact that wounds should be pro- 
tected to prevent infection, there will be less trouble from wood-destroy- 
ing fungi. 

In certain cases the fungus gains an entrance to the tree through im- 
proper pruning. If short spurs of the branches are left, the tissue dies 
back to the main axis and the fungus finds in these dead spurs an easy 
lodgment. . The accompanying illustration of a cherry branch shows such 
spurs and the sporophores of a fungus which entered through them. 



50 



University of Texas 



The broom-like tufts of branches which appear on the silver fir, cedar, 
birches, cherry, and other trees are popularly called witches' brooms. In 
the majority of cases a rust fungus is the cause of the abnormal prolifer- 
ation, and the resulting branches generally stand more or less erect, 
giving the characteristic broom-like effect. Insects may in some cases 
be the cause of witches' brooms, but there are numerous instances in 





Fig. 51. — Sporophores or fruiting bodies of a wounLl fungus on the cherry. 

Original. 



which the causal organism has not yet been observed. The witches' 
broom of our common liackberry is due either to an insect alone or to an 
insect and fungus working together. Cockscomb-like growths are closely 
related to witches' brooms in their manner of formation. In growths of 
this sort on the gooseberry, a louse and a mildew are quite constantly 
present. 



SyiiiiitoDis of Disease in Plants 



51 



While the formation of rosettes is the normal form of growth in many 
plants, such as dandelion, shepherd's purse, lettuce, and others, the ap- 
pearance of rosettes or the assumption of the rosette hahit is a sure in- 




Fi„ 52.— Sporophores of fundus ( Schizophyllum commune) on cherry. Show* 

the short spurs left by improper pruning, which allowed the entrance 

the fungiis. Original. 

dication of disease in several cases. Very good examples of this symp- 
tom are to be observed in the "rosette disease" of the potato and in 
the ''peach rosette." Both of these diseases are at present confined to the 
eastern and southern part of the United States. 



52 



University of Texas 



The root system of a plant may also show an abnormal abundance of 
branches giving rise to a tnfted mass of fine fibrous roots. This condi- 




Pig. 53.— Cherry tree showino- "hairy root." The shoot also shows a marked 
rosette arranoemeut of the leaves. Original. 



tion is designated as the "hair}^ root," and it may be accompanied by an 
abnormal aggregation of the leaves and l)ranches in the shoot. This is 
well shown in the accompanying illustration of a young cherry tree. 



Sijiiijitoins of Discdsr in Plants 



53 



There are niaiiy di?ea,-'es in which tlie oiittiow of a slimy, gummy, or 
resinous suhstanee is a characteristic symptom. In tin; so-called "slime- 
flux" of deciduous trees the exudation is semi-fluid in nature and does 
not set into a solid sul)stanee with the accumulation of the excretion. 
This symptom may indicate a parasitic organism or it may he due to deep- 




Fig. 54. — Normal ear of corn and one all'ectcd witli dry-rot (Diplodia zeae), 

Oi'iginal. 



seated physiological disturhances induced hy other factors. In conifer- 
ous trees in which an excretion of resin is a normal phenomenon, we often 
find an ahnormal increase as a result of disease. One of the first symp- 
toms attending the presence of wood-destroying fungi in our forest ever- 
greens is this ahnormal production of jDitch or resin. This symptom is 



54 University of Texas 

frequently noticeable before the sporophores of the fnngus make their ap- 
pearance on the surface of the trunk. The production of a clear or 
amber-colored secretion which accumulates upon the surface of trunk, 
branches or even small twigs is characteristic of the "gummosis" of cher- 
ries and other stone fruits. This material generally sets into more or 
less solid nodular or flattened masses, and the fruit itself often shows 
numerous "gummiperlcn." In peaches and plums the fruit from affected 
trees often shows a splitting apart of the halves of the pit or stone. In 
general, however, cherry trees and peach trees suffer more from gummosis 
than any other stone fruits. In some cases gummosis is induced by 
wounds; in others it is a symptom attending the presence of a fungus 




Fig. 55. — Cross scclidn of t.lic tnui oi aii c,^-- plant rotted by a fungus 
(Phyllosticta hortorum). Original. 

parasite; while in many instances it is due to a deep-seated disturbance 
of the nutritive processes which results in the decomposition of more or 
less extensive masses of tissue. Some of the external factors which espe- 
cially favor or predispose to this trouble are heavy soils, high levels of 
ground water, deep planting, and transplanting at inopportune times. 

The rotting of parts or organs of a plant, the "gangrene" of plant tis- 
sue is always due to the presence of some organism, either a fungus or 
bacterium. In some cases there may be a slow decomposition of the 
tissue without any liquefaction or softening of the atfected parts. In 
these "dry rots" the parasitic oi'ganism gradually digests cell-wall or re- 
serve food materials, or both, with the result that living cells are deprived 



Sympto7ns of Disease in Plants 



55 



of their necessary food, and succumb or arc directly attacked, wliile mc- 
clianical tissues are weakened and fail to perform their normal functions. 




Fig. 50.— A root-rot of cotton. Plants show a marked constriction of the stem 

and numerous wart-like pustules. Photograph by 

F. D. Heald and F. A. Wolf. 

In other cases the rotting may be fairly rapid, with more or less liquefac- 
tion and softening and discoloration of the affected parts. In these 



56 



Universitij of Texas 



"soft-rots" the fungus or bacterium generally spreads rapidly through 
the affected parts in case favorable conditions prevail. 

There are no parts of a plant that are exempt from rotting, but in gen- 
eral the dormant or languid parts are more likely to suffer from the in- 
roads of rot-producing organisms. ISTearly all the troubles in which rot- 
ting is a characteristic symptom may be grouped under the head of root- 
rots, stem or trunlv rots, bud rots, and fruit rots. 

Plants with Heshy or woody roots often have their root system invaded 
by parasitic or semi-parasitic organisms, which work in such a way as to 
give rise to disease generally characterized as root-rot. Woody roots 
generally undergo such changes that the trouble is characterized as a dry 



;ijr<-^*?^'^!WH#455»iy.' V- "■—^ 




Fig. 57. — ^Cross section of the trunk of a catulpa tree. Central portion has been 
disintegrated by tlie action of a fungus (Polyporus versicolor). Original. 



rot, Avhile fleshy roots tliat are gorged with reserve food material often 
of a saccharine nature are more likely to be affected l^y organisms caus- 
ing a soft rot. Our root crops, such as beets, turnips, carrots, and sweet 
potatoes, are often affected. Sometimes the organism tliat causes the 
rotting gains an entrance while the roots are still growing in the field or 
garden, and continues the work of destruction after the crop is harvested 
and in storage. At other times the rot-producing organisms only gain 
an entrance after the crop is harvested and in storage or while the roots 
are in a dormant or languid condition and so less able to withstand the 
attack. The soft rots of beets, turnips, and carrots are frequently due to 
bacteria, while the soft rots of sweet potatoes are of fungus origin. Much 



Sijiiiptonis of Disease in Plants 



57 



can be done io prevent tliosc troiil)les l^y Droper nieasuTcs in the field and 
storage nnder conditions that are unfavoral)le for the growth of either 
bacteria or fnngi. 

i^lfalfa. clover, and cotton often sntl'er from root- rots to such an extent 
as to cause serious loss. In one such disease of alfalfa the alfected field 
will show small circular patches of dead plants which increase in circum- 
ference as the fungus advances through the soil. In another case the 
dying and dead plants are scattered throughout the field and show a rot- 
ting of the root of a different character. The evidence at hand leads to 




Fig. .58. 



-Portion of a slab of cottonwood, showinn; the effect of a wood-destroying 
fnnoiis {>]]fvinoia nieoalonia) . Orioinal. 



the opinion that this last type of root-rot is induced primarily by eitlier 
unfavorable soil or climatic factors, or both, which so lowers the tone and 
vigor of the plants that various saprophytic forms come in and complete 
the work of destruction. Tm'o different root-rots of cotton are known. 
One of these is very common in Texas and has received the common 
name of the "Texas root-rot." It is not uncommon to find fields in 
which 25 to 75 per cent of the crop has been ruined l)y the inroads of 
these root-rotting parasites. 

Some of our fruit trees, such as the clierry and apple, and also forest 



58 



University of Texas 




Symiitoms of Disease in Plants 



59 



and shade u'ee?, are attacked Ijy root-rotting fungi. TIio wood of root 
arid even the crown is slowly disintegrated, and the mechanical structures 
are so weakened that severe storms may cause the affected tree to be 
blown over. White strands or fibrils, the vegetative body of the fungus, 
may sometimes be seen upon the fractured surface. In other cases the 
aU'ected tree may remain standing until it finally succumbs to the attack. 
Modified stems, like tubers, rhizomes, bulbs, or corns, which serve as 
storage organs for reserve food, suffer in many cases from rot-producing 
fungi or bacteria. These storage organs are affected in much the same 




Fig. CO. — Aipple rotted by black mold (Rhizopus nigricans). Specimen was 

placed in a damp chamber to cause the mold to grow over the 

surface. Original. 



way as the fleshy roots, but dry rots are not uncommon. The potato is 
affected by several diseases in whicli a soft rot of tlie tubers is a marked 
symptom. This is true in the disease known as tlie late blight, a fungus 
disease, and also in the bacterial blights, while in some cases a soft rot 
may result when the tops are not atfeeted. In the dry-rot of the potato 
the fibro-vascular ring often shows a conspicuous darkening when the 
tubers are cut across, and the badly rotted portions become shrunken 
and darkened and more or less corroded. Diseases of this kind may be 
spread by planting affected seed. Iris rhizomes, canna root-stocks, 



60 



University of Texas 




Symptoms of Disease in Plants 61 

liyacinth or ealla lily bulbs, onions and similar niodified storage oro-ans 
are often affected by organisms which produce soft rots. 

An immense amount of loss is caused in standing timljer and in fruit 
trees from dry-rots of the trunk due to wood-destroying fungi which gain 
an entrance through wounds or otherwise. It frequently happens that 
the trunk of a tree is thoroughly permeated by the vegetative hijphae of 
the fungus l)efore any external evidences of its presence can be detected. 
These wood-destroying iimgi may give rise to external fruiting bodies, 
either toadstool-like or bracket sporopliores, when they have reached a 
sufficient vigor of growth. The affected wood is gradually disintegrated 
hy the work of the fungus which digests the woody elements. It is thus 
transformed into a brittle or even punky condition that renders it unfit 
for the purposes wliieli it must serve as a functional part of the tree, and 
lessens or destroys the value of the wood for hunber. "Wood affected by 
■dry-rot is often discolored, sometimes showing a marked blue, pink, yel- 
low, or reddish-brown coloration. Black lines may often be noticed cross- 
ing irregularly through the wood or surrounding definite areas. The 
vegetative body of the fungus is frequently invisible to the naked eye, 
hut in some cases it shows as tine strands or ropes, or even lamellae that 
run between the wood iiljers or along the silver grain. 

In a few cases bvtds are affected by rot-producing fungi or bacteria. In 
the tropical regions the cocoanut is seriously affected by a bacterium 
which causes a rotting of the large terminal bud. In this connection the 
black rot of the cabbage may be mentioned, for the cabbage is really a 
large bud. This disease is of bacterial origin, the organism entering at 
the leaf margins. They make their way down through the veins to the 
central axis and frequently cause a complete rotting of the fully or par- 
tially developed heads. This disease is particularly destructive on low, 
heavy soils. One of the l)est illustrations of a fungus bud-rot is to be 
observed in a serious disease of certain varieties of greenhouse carnations. 
In this disease the fungus may gain an entrance into the bud at an early 
stage of its development, and cause a complete rotting of the petals be- 
fore the calyx has opened. In other cases the rotting starts later and the 
flower may be partially opened before its development is arrested. In 
these affected flowers the claws of the petals will be bro^ra and soft even 
though the exposed portions are normal in appearance. A minute mite 
is constantly associated with this disease and the evidence points to the 
fact that the mite acts as a carrier for the fungus, transferring it from 
bud to bud. 

The rotting of fruit is always caused l)y the inroads of bacteria or 
fungi. In the majority of cases it is some filamentous fungus that causes 
the trouble. The fruit-rotting fungi may gain an entrance through 
'^vounds or bruises, or in other cases they may penetrate the uninjured 



62 



University of Texas 




Fig. 62.— Normal Belinower apple and coal black "mummy' produced by brown 
rot fungus (Sclerotinia fructigena). Original. 



MB Sf 



Symptoms of Disease in Plants 63 

skin. In some eases the rotting may Ijegin wlien the fruit is still imma- 
ture and hanging on the tree, and the work of destruction may be con- 
tinued after the crop is harvested. Many fruit-rotting fungi only at- 
tack the ripe fruit, just at time of nuiturily or after it has been harvested 
or during storage. Soft fruits, like strawberries, suffer rapid decay 
from fungi, while fruits like apples with a protective epidermis may be 
preserved for a long time. 

The brown rot of peaches, plums and cherries is very destructive in 
certain regions when conditions become favorable. Affected, fruits show 
brown patches of soft tissue. The patches spread rapidly in circumfer- 
ence until the whole fruit is involved, and if the weather is moist these 
rotted areas will show conspicuous pale brown conidial tufts which pro- 
duce myriads of spores that spread the rot to other fruits. It is these 
rotting fruits that produce the "mummies" already mentioned. A 
fungus similar to that which cau'^es the brown rot of peaches, plums, 
and cherries also affects the apple, it may work in the same way, 
or it iuay transform the apple into a blaek mummy, shiny and coal 
black, with no external evidence of the fungus. This rot of the apple 
should not be confused with the true black rot which works in a 
somewhat similar way. In the true black rot the surface of the apple 
soon becomes covered with minute black pustules and the fruit be- 
comes more or less shriveled and shrunken. In both the black rot 
and the brown rot the pulp is colored a dark browai, the black coloration 
being due to changes in the skin. The bitter rot shows brown circular 
areas with concentric zones of pink spore-pustules. This is one of the 
most destructive apple diseases known for the central Mississippi Valley 
region, especially for southern Illinois and Missouri and adjacent terri- 
tory. Green mold is one of the most destructive fungi for stored apples. 
Apples affected by this rot often show only brown patches in wdiich the 
tissue becomes rapidly softened and the whole apple transformed into a 
soft rotten mass. In moist conditions the bluish-green spore-tufts of the 
fungus may be seen upon the surface of the fruit. In addition to the 
various rots mentioned, a white rot of the apple has been recently ob- 
served in which the entire pulp may be transformed into a soft mass 
without any discoloration. By paying attention to proper spraying to 
prevent insect injuries and other fungus diseases, by careful handling in 
harvesting, and by storage under conditions that retard the growth of 
fungi, much of the trouble from rotting of apples can be prevented. 




























O V 



■/ ^o.'-^-/ \'^-^\/ -o^^^-,0' 



A. ^ *y^^- 

,0'' 






A 






.^'"'-. 



t^o^- 














^^--^ 



^ 



o V 





°o> 



.^^ 






^^-n 



*1 C» *! 



•1 o> 



LIBRARY OF CONGRESS 



002 81 1 462 A # 




